Top reading rules are gaining considerable acceptance because of the ease and convenience with which they permit inside dimensions to be measured. It is of considerable importance, in such a rule, that the measuring blade be disposed in a substantially rectilinear condition when passing beneath the window provided in the top wall, so that readings taken will be accurate. This in turn imposes certain constraints upon the internal system by which the blade is guided, since it means that relatively severe directional changes must occur at the points of transition to and from the section of the path that traverses the window.
Additional factors must also be taken into account in designing the blade guiding and supporting system. It is of course of utmost importance that the components of the system minimize the amount of friction created, to avoid undue blade wear and to ensure smooth and trouble-free operation of the rule. This has been achieved by utilizing small guide rollers supported within the casing at transition points along the path of blade movement, which will generally be fabricated from a synthetic resinous material having a low coefficient of friction; coatings of solid lubricants on blade surfaces have also been used to facilitate movement.
Another consideration is the external size of the rule casing. Not only is it desirable, from the standpoint of manufacturing and shipping expense, to make the rule as compact as possible (consistent with practicablity and convenience of handling) but indeed, it has been found that the consumer has developed an unconscious expectation concerning the "feel" of the instrument; as a result, even a slight enlargement of the size of the casing has met with a degree of consumer resistance.
Although top reading rules (as well as other coiled measuring rules) may be designed so that the blade is manually withdrawn and returned to the spool, in many instances a power-return feature will be preferable. However, the blade and internal components of such rules are subjected to substantial levels of shock, due to impact of the hook member upon the casing when the blade has been fully retracted. Not only does this require that the parts of the unit be so constructed as to withstand the forces involved, but it has now also been appreciated that there is a tendency for kinking of the blade to occur when movement of the blade under the force of the power spring is abruptly arrested; this is especially so in the top reading rules, due to the sharp directional changes that the blade must make in order to pass rectilinearly under the window.
It has been determined that the tendency for such kinking to occur is due to a number of factors. One is of course the force that is imparted to the blade by the power spring, since that will in turn determine the inertia of the blade and the energy that must be dissipated upon impact. The construction of the blade itself will also be a major determinant; if it is relatively wide and/or made of a material having a high yield point (i.e., tending to resist a permanent set upon distortion), the likelihood of kinking will be reduced commensurately. A third primary factor concerns the severity of deflection to which the blade will be subjected when taut. This is most important at the point along the path of movement closest to the spool, since the segment of the blade in that region will have the highest energy level when movement is arrested.
It is known that the transversely curved metal blades used in measuring rules of this sort exhibit a so-called "natural curvature" when deflected longitudinally upon themselves, the radius of which curvature is substantially independent of the degree of deflection; this characteristic is described, for example, in Quenot U.S. Pat. No. 3,494,038. As long as the blade is subjected to no deflection sharper than the radius of natural curvature, it is evident that kinking will not occur. Hence, any such tendency could be eliminated in a measuring rule by utilizing guide rollers of sufficient radius.
As mentioned above, however, space limitations prohibit the use of overly large rollers, and this is particularly true when other elements must be accommodated within the casing, such as the slide-type locking members which are widely employed to maintain extended rule positions. Similarily, the use of a series of relatively small rollers, arranged to constrain the blade to a path which simulates the natural curvature (as is also disclosed in the Quenot patent), is not practical because of the deviations from conventional casing size and design that it requires, and because the added rollers produce excessive levels of friction.
In addition to the Quenot patent mentioned above, the following United States patents may be of interest to the instant subject matter:
Volz U.S. Pat. No. 2,207,277 shows the use of ribs which function as guide surfaces in a coilable measuring device. Anderson U.S. Pat. No. 3,255,531 discloses a top-reading measuring rule wherein ball bearings, which may be seated in arcuate plates, define means for guiding the tape through the housing. A top-reading tape measure is shown in Maksim Jr. U.S. Pat. No. 3,281,943, which uses a straight post member to facilitate measuring irregularly shaped objects.
Quenot U.S. Pat. No. 3,493,190 shows multiple guide elements for the blade of a top reading, power return rule. The rollers cooperate with a rib to guide the tape along a constant "trajectory"; two continuous ribs on either side of the tape, and various other combinations of elements, can be substituted. Finally, the power-return, top-reading tape measuring devices disclosed in King U.S. Pat. No. 3,731,389 has an integral marker and uses curved plates to guide the tape.
Accordingly, it is the broad object of the present invention to provide a novel top reading power return rule in which the possibility of blade kinking, upon abrupt arrestment of retraction, is substantially eliminated.
It is a more specific object of the invention to provide such a rule in which kinking is avoided without increasing the size of the casing or the amount of friction on the blade.
An additional object of the invention is to provide such a rule wherein the foregoing features and advantages are achieved without significant increase in the cost of complexity of manufacture.